This invention relates to metal encapsulated, pressurized gas insulated high voltage switching apparatus in general and more particularly to disconnect switches, of the type having two field electrodes which define, in the "off" position, a switching gap between two potentials, and having a movable switching rod which crosses the switching gap in the "on" position and together with which the field electrode which surrounds the rod coaxially can be moved into an intermediate position which favors the electric field between the two field electrodes during the closing process.
From DE-OS No. 27 11 166, a metal encapsulated, pressurized gas insulated grounding switch for metal encapsulated high voltage switching installations is known, the switching rod of which is coaxially surrounded by a field electrode. During the closing process, this field electrode is moved in the same direction as the switching rod and transferred to a position closer to the mating stationary contact. Thereby, the breakdown spacing is reduced in this known high voltage switch. The effect of the field electrode is to delay the firing instant of the arc. The movement of the field electrode is accomplished by friction at the switching rod; it accordingly depends on factors determining the friction contact such as, for instance, heating, wear and manufacturing tolerances. Due to the free movement of the field electrode between two fixed stops chosen for the motion, the electrode is carried along even at the beginning of the switching off process when the grounding switch is opened.
In high voltage switch gear such as disconnect switches, which switch at velocities of about 4 cm/sec, the movable switching rod is, as a rule, actuated by a motor drive. The switching on time is then between 5 and 10 sec, depending on the size of the switching gap which depends on the rated voltage. If disconnect switches of this type are switched on, a pre-breakdown with a subsequent low current arc always occurs if the disconnect switch is switched under voltage. The duration of the preliminary arc can then be several seconds, depending on the circumstances.
In the known designs of disconnect switches which are suitable for voltages up to 245 kV, this breakdown gap is so small, especially in sulfur hexafluoride which is used as the quenching and insulating medium in metal encapsulated installations, that the arc is not expected to wander off while it is burning.
In disconnect switches for voltages higher than 245 kV, the pre-breakdown length of the arc is larger because of the longer switching gap so that the danger exists that the arc, while it burns, will travel away from its point of origin and settles at the grounded encapsulation of the installation.
When disconnect switches are opened, arcs that can similarly wander off can occur during the opening operation, for instance, due to unavoidable charges on the connected transmission lines, especially cables. If the arc wanders away from its point of origin and settles at the grounded encapsulation of the installation, the danger exists that parts of the installation will be destroyed due to the then existing short to ground.